Several types of earth boring drill bits with rolling cutters are known in the art, for use in oil and gas well drilling, water well drilling, and mining. Such drill bits can have a body fitted with one or more rotatable cutters that rotate as the bit is rotated in a borehole. The rotatable cutters can be conical, sometimes called rolling cones, or some other shape. The cutting elements of such bits can be teeth machined or otherwise integrally formed on the cutters, or hard metal inserts mounted on the cutters.
An earth boring drill bit is typically mounted on the lower end of a drill string of pipe, and the pipe is rotated by machinery located at the surface of the drill site. As the pipe is rotated about a substantially vertical axis, the drill bit enters the ground, as weight is applied, and proceeds along a planned path toward a target. The drill path can deviate appreciably from vertical, but for purposes of this disclosure, such deviations are not material.
The drill bit will typically have a number of teeth or cutting inserts which are shaped and positioned to disintegrate the earth formation along the planned drill path, in forming the borehole. The teeth or inserts which disintegrate the formation are generally located in the area of the lower end of the drill bit, which is the front portion of the drill bit as it proceeds through the formation. The borehole formed in the drilling process will have a diameter roughly equal to the diameter of the drill bit. As drilling progresses, the drill bit will proceed along the borehole, and the sides or periphery of the drill bit above the lower end will slide past the sidewall of the borehole.
The formation through which the drill bit passes can often be very hard and abrasive material, so the periphery of the drill bit can suffer significant erosion and wear from contact with the sidewall of the borehole. For this reason, it is common to protect the peripheral surfaces of the drill bit from erosion and wear by several means. The surfaces can be protected by hardening the steel or by depositing very hard material called hardfacing, such as tungsten carbide, over the surfaces. Some surfaces are protected by installing buttons or inserts of very hard and abrasion resistant material in holes in the surface, with an end of each insert either substantially flush with the surface or raised slightly above the surface. The use of such inserts can also assist in maintaining the full design diameter of the borehole, as the primary cutting inserts or teeth wear down. It is important to maintain the full gage of the hole to permit eventual insertion of casing and to avoid drag and damage on the following bit.
These gage inserts can be made of tungsten carbide, natural diamond, synthetic diamond, or composites of these materials. They can have an outer surface ground to conform to the shape of the peripheral surface to be protected, or they can have an outer surface with another contour, such as flat. A peripheral surface on the drill bit which is protected by hard inserts is generally designed to be close to the borehole wall at or near the full designed diameter, or full gage. These surfaces are therefore typically referred to as gage surfaces of the bit.
The gage surface can be an inclined surface at the base of a rotatable cutter. If the rotatable cutter is conical, the gage surface will typically be a frustum of a cone, called a frusto-conical surface. In any case, the gage surface is one which is designed to generally align with the sidewall of the borehole, as the drill bit rotates. A gage surface on a rotatable cutter would only align with the sidewall over a narrow area where the cutter contacts the sidewall, in a generally sliding fashion, as the drill bit rotates and advances.
When a hard material insert is used on the gage surface of a drill bit in hard and abrasive formations, it often happens that the insert experiences rapid deterioration or even premature failure. In a tungsten carbide insert, the deterioration usually takes the form of heavy wear and heat checking as the insert conforms to the sidewall of the borehole. However, in the harder inserts, such as those containing diamond materials, the insert will not wear down. It will instead continue to stick out from the gage surface, causing it to be severely loaded. This results in overheating, checking, cracking, chipping, and eventual failure. This deterioration usually initially takes the form of heat checking, often followed by cracking, on the outer surface of the insert, localized in the portion of the surface that can be called the "trailing" area as opposed to the "leading" area. The area characterized as the leading area is that area of the outer surface or face of the insert that first engages the sidewall of the borehole as the bit rotates. The area characterized as the trailing area is that area of the outer surface of the insert that lies on the opposite side of the outer surface from the leading area. Similarly, the edge formed at the intersection of the outer surface and cylindrical body of the gage insert that first engages the sidewall is commonly called the leading edge and the opposite edge is called the trailing edge. This edge may be a chamfer connecting the outer surface and the cylindrical surface.
The leading edge of the insert has an angular orientation about the axis of the insert that is influenced by a number of factors including weight on bit, bit r.p.m., and others. The combination of these factors will result in a characteristic angular orientation of the leading edge of the insert. This angular orientation will typically vary from alignment with the lowermost edge of the insert, as the bit hangs vertically, to an orientation that can be rotated as much as 90 degrees from the vertical, in the direction of bit rotation. The direction of bit rotation is typically clockwise, as viewed from the top of the borehole. Therefore, in other words, depending upon a number of factors, if one views the outer end surface of the gage insert on a vertical bit, the gage insert will typically first engage the sidewall somewhere in the lower left hand quadrant of the surface.
Heat checking has frequently been observed in the trailing area of the gage insert outer surface. It can progress to cracking and rapid erosion or chipping away of the insert material. It is thought that this damage results from the generation of excessive heat by the sliding under high load of the entire outer surface of the insert along the sidewall of the borehole. In other words, since the gage insert in known drill bits is installed with its outer surface generally aligned with the sidewall of the borehole, excessive friction generates excessive heat at the surface of the insert. This excessive heat generation and the resultant heat damage are more pronounced in the trailing area of the insert surface, probably since the material being crushed by the leading half of the insert is extruded underneath the insert and further increases the contact stresses as the insert advances. As the insert is exiting its contact with the borehole wall, excessive tensile forces act on the trailing edge, causing propagation of the heat cracks. In addition to heat checking of the insert and associated damage, the excessive heat generated can have deleterious effects on other nearby components of the drill bit, such as the elastomeric seals in a rotatable cutter.
It is an object of the present invention, therefore, to develop a drill bit which will not develop excessive heat and the related damage in the vicinity of the gage surfaces, by improving the interaction between the gage inserts and the sidewall of the borehole. It is a further object of the present invention to develop a drill bit which will have gage inserts that are not aligned parallel to and substantially flush with the sidewall of the borehole being drilled, but which will have the trailing area of the outer surface of the insert recessed from the sidewall. It is a still further object of the present invention to develop a drill bit which will avoid the aforementioned problems of insert failure, and which will be relatively cost effective to manufacture.